The present invention relates to a surface acoustic wave convolver arrangement wherein an integration electrode is provided on the substrate acting as a waveguide. Interdigital input transducers are provided along with the integration electrode.
Convolver arrangements which operate with surface acoustic waves (SAW) are known from the prior art. Reference is made in this regard to the following publications: 1. IEEE Ultrasonics Symposium (1981), page 186, ff.; 2. IEEE Ultrasonics Symposium (1980), page 59 ff.; 3. IEEE Transactions on Sonics and Ultrasonics, Volume 30 (1983), page 43 ff.; 4. EU-A 00 57 332, all incorporated herein by reference.
From publication 1., an arrangement with the integration electrode provided on a substrate wherein the integration electrode functions as a waveguide. At ends of the waveguide first and second input transducers are provided. The input transducers are interdigital finger structures with bus bars. Publication 1 discloses and illustrates an integration electrode designed as a waveguide which is a continuous metallization coating of the surface of the substrate member not illustrated therein. Opposite the right and the left end of the waveguide, one input transducer each is illustrated whose aperture, given by the overlapping length of the interdigitally arranged fingers of this transducer, is equal to the width of the respective opposite end, i.e. is equal to the width of the metallization strip. As is apparent, the two input transducers have nonequidistant spacings of the interdigitally arranged fingers, corresponding to the necessary dispersion property of the transducers. The dispersion of the two transducers can be selected to be of varying size, whereby a dispersion of the waveguide can be compensated.
With a convolver arrangement as described above and also with other convolver arrangements, in the case of one input signal each which is applied to the respective input transducers, in the region of the integration electrode a mixed product of the input signals can be produced which--as is illustrated in publication No. 1--can be taken from the integration electrode. A prerequisite for the occurrence of such a mixed product is that in the region of the integration electrode, a non-linear property for the sonic waves travelling therein produced by the respective input transducer be present. This signifies that the input transducers, approximately comparable to other surface acoustic wave arrangements, must produce a very high-intensity sonic wave in the surface of the substrate in the region of the integration electrode. Accordingly, for such convolver arrangements, the problem exists of being able to introduce high-intensity surface waves produced in the respective input transducer into the opposite end of the waveguide of the integration electrode. This comprises the additional problem of generating a high-intensity sonic wave in the transducer. For this purpose, a pre-condition is that the transducer be optimally adapted in terms of impedance to the impedance value which is usually specified at the output of that particular electronic circuit to which the respective input transducer is connected.
The impedance value of an input transducer as employed results essentially from its radiation resistance and its interelectrode capacitance. For this reason alone, the respective input transducer cannot be arbitrarily dimensioned. On the other hand, the wave transmitted by the transducer into the integration electrode must strike the wave in as optimum a fashion as possible, i.e. the transmitting characteristic of the respective transducer must be matched to the receiving characteristic of the opposite end of the integration electrode.
For the arrangement illustrated in of publication 1., through correspondence of the apertures, this above-stated matching is to be effected. Through the design of the input transducer as a dispersive transducer, the demands of the generation of intense sonic waves (through corresponding lengths, or finger number, respectively, of the transducer) can also be complied with.
In the arrangement illustrated in publication 2., this problem is solved through use of input transducers together with beam compressors, separated therefrom. The input transducers have very long fingers so that the aperture of the input transducer is far greater than the aperture of the integration electrode. The above-described beam compressor is intended to match the very different apertures to one another.
Publication 3. describes, and illustrates, a convolver arrangement with input transducers which have curved finger electrodes. The finger electrodes have a uniform curvature which is so selected that the respective input transducer has a focusing property for the sonic wave generated in the transducer. The length of the individual interdigitally arranged fingers of the respective input transducer is uniformly great. The arrangement requires a considerably large surface.
Publication 4., illustrates a convolver arrangement not relevant per se for the invention but which is described in the following. Finger electrodes are shown therein which, instead of a rectilinear shape, have a finger shape which results in a certain finger offset. However, it must be pointed out that, in the region of the aperture of the integration electrode, the fingers which, as a whole, are not rectilinear, are still rectilinear and the bends of its shape lie only outside the aperture. Bent fingers also occur only in the case of the beam compressor. The input transducers have the conventional rectilinear design of the interdigitally arranged fingers.
Initially, the problems were already pointed out which arise in the case of a convolver arrangement from the demand for generating high-intensity sonic waves on a narrow aperture (of the integration electrode) in a concentrated fashion.